1. Field of the Invention
This invention relates in general to means and methods for holding electronic substrates, and, more particularly, to improved means and methods for holding and/or spinning semiconductor wafers on vacuum chucks, especially those used in wafer processing and testing.
2. Background Art
Vacuum chucks are much used in the manufacture of electronic devices, particularly for electronic devices which are processed in the form of this wafers or substrates. Familiar examples are semiconductor devices, integrated circuits, hybrid circuits, bubble memory circuits, and Josephson device circuits. The substrates used for the construction of these types of devices or circuits are generally in the form of substantially flat thin wafers made of semiconducting or insulating materials. As used herein, the word wafer is intended to refer generally to thin substrates used for forming various types of electronic devices.
As a part of the manufacturing process for these types of electronic devices and circuits, it is common to apply a variety of different layers to the wafer surface. Photoresist masking techniques are often used to pattern these different layers. The photoresists, as well as other materials, are applied by first placing a small droplet of the material on the wafer surface and then spinning the wafer at a high speed so that centrifugal force spreads the droplet out into a thin uniform layer. The wafer is generally held in place on the spinner head or chuck by means of a vacuum applied to holes or grooves in the face of the chuck against which the wafer rests. Spinning apparatus of this sort is well-known and commerically available.
In a typical spinner apparatus, the wafers are rotated at a high rate of speed, typically in the range 1000-10000 RPM. These high rotational speeds create large sheer forces during the spinning operation which tend to throw the wafer off the chuck. The more imperfect the centering of the wafer on the chuck and the more irregular the shape of the wafer, the greater the forces which the wafer will experience during the spinning operation. A common failing of prior art spinner apparatus has been the inability of the spinner chuck to reliably hold the wafers in place during the spinning operation. Some fraction of the wafers placed on the spinner chuck fly off and break during processing. Accordingly, there is a continuing need for improved spinner apparatus which exhibits a reduced incidence of wafer breakage.
Prior art attempts to improve wafer retention on spinner chucks have involved the use of O-rings at the periphery of the chuck to improve the vacuum seal between the wafer and the surface of the chuck. The greater the vacuum which can be maintained during the spinning operation, the greater the holding power of the spinner chuck, and the lower the probability of wafer fly-off and breakage.
However, conventional O-ring seals between the wafer and the spinner chuck have not proved satisfactory. The presence of the O-ring introduces other forms of yield loss which negate the advantage of the increased holding force. The presence of the O-ring prevents the wafer from being uniformly supported on the surface of the chuck. As a consequence, when vacuum is applied, the very thin wafers bend rather than compressing the O-rings. Some wafers will break when bent. Even if they do not break, the bent shape degrades the uniformity of the layer being spun. Further, prior art chucks, with or without O-rings, function poorly when the wafer is not perfectly flat.
Accordingly, it is an object of the present invention to provide an improved means and method for obtaining reduced wafer breakage in spinner apparatus.
It is a further object of the present invention to provide an improved wafer chuck having greater wafer retention force.
It is an additional object of the present invention to provide an improved means and method for obtaining a vacuum seal between wafers and a platform for supporting wafers without introducing significant wafer breakage and uneven distribution of spun coatings.
It is a further object of the present invention to provide an improved means and method for an O-ring sealed vacuum chuck in which the wafer is supported by the chuck surface and not the O-ring.
It is an additional object of the present invention to provide a moveable seal between the wafer and chuck which can better accommodate non-flat wafers.